Thermoplastic olefin polymers prepared by physically blending or mixing monoolefin copolymer or terpolymer rubbers and polyolefins are known (see, e.g. U.S. Pat. Nos. 3,036,987, 3,835,201 and 4,748,206). However, in order to achieve a good dispersion of the rubber in the polyolefin it is necessary to employ energy intensive mixing.
The formation of thermoplastic elastomers (TPE) from such blends is also known. Although there are a number of methods taught, the one generally practiced is dynamic vulcanization of such blends, such as by the method described in U.S. Pat. No. 3,806,558.
In order to avoid the disadvantages associated with physical blending while at the same time avoid the necessity to dynamically vulcanize such blends, efforts have been made to produce reactor or chemical blends of a crystalline polypropylene and an amorphous ethylene-propylene copolymer rubber by sequential polymerization in a reactor.
In U.S. Pat. No. 4,489,195, for example, the preparation of polyolefin thermoplastic elastomers by a two-stage polymerization process using stereospecific catalysts composed of an organoaluminum compound and a solid catalyst component on a magnesium halide support is taught. In the first stage 5-50 wt. % of a homopolymer of propylene is formed, and in the second, 50-95% of an ethylene-propylene copolymer having a propylene content of 5-60% is prepared by adding ethylene monomer which reacts with the unreacted propylene monomer of the first stage. The polypropylene produced in the first stage and the ethylene-propylene rubber of the second stage are believed to be chemically combined so as to form a block copolymer. One of the disadvantages of this method is that the temperature in the second stage must be kept relatively low, i.e. not more than 50.degree. C., in order to prevent agglomeration of the ethylene-copolymer rubber particles and reactor fouling. This need to operate the second stage at relatively low temperatures penalizes the process with respect to heat exchange and diminishes catalyst mileage.
U.S. Pat. No. 4,491,652 also describes the preparation of polypropylene thermoplastic elastomers in two stages. In the first stage the propylene is polymerized to a homopolymer polypropylene. In the second stage, ethylene is added and ethylene and propylene are polymerized in the presence of a solvent, preferably at temperatures of 60.degree.-77.degree. C., to form rubbery copolymers and block copolymers of the polypropylene and ethylene/propylene rubbery copolymer. The polymerizations conditions employed in the second stage leads to the formation of a partially soluble rubbery copolymer which tends to cause the resultant product to lump or agglomerate. These lumps or agglomerates must be broken up to provide a homogeneous product. Typically this is done by grinding on a mill. As a matter of fact, it is known that when over 20%, based on the thermoplastic elastomer, of the rubbery ethylene/propylene copolymer is produced during the preparation of the thermoplastic elastomer, it is impossible to avoid agglomeration of the particles even when the polymerization takes place in the presence of stereospecific catalysts (see, e.g., European application 0029651 and U.S. Pat. No. 4,259,461).
Polymerization of such rubbery copolymers in a gas process, even in small amounts of 20% or more, likewise leads to product agglomeration and fouling of the reactors. This reactor fouling effectively prevents one from conducting such a polymerization process in gas phase.
Therefore, it is necessary to be able to produce a thermoplastic olefin polymer having the desired balance of mechanical properties in a reactor or sequence of reactors, including, where desirable, at least one gas phase reactor, which avoids the disadvantages associated with the present methods of producing this type of polymer.